This invention relates to a process for producing an aqueous formaldehyde solution having a particularly low methanol content by catalytic reaction of a feed gas containing methanol, air and water over a silver catalyst.
According to the conventional process for producing formaldehyde by oxidative dehydrogenation of methanol, a feed gas containing raw material methanol, air and steam, and further containing, if necessary, a waste gas, etc. is subjected to reaction over a silver catalyst, while recovering the heat of reaction from the resulting product gas in a boiler, if necessary, and the product gas is led to an absorption column to recover formaldehyde in the form of an aqueous solution. In that case, the methanol in excess over the air is used, and thus the resulting formaldehyde is usually contaminated with the unreacted remaining methanol. The remaining methanol itself has a stabilization effect on the formaldehyde solution, but the mere presence of methanol in excess is useless, and thus it has been now keenly desired to produce an aqueous formaldehyde solution having a low methanol content.
To this end, it is the ordinary expedient to increase a molar ratio of air to methanol to about 1.8, thereby decreasing the amount of remaining methanol in the product gas and obtaining formaldehyde having a low methanol content. In that case, steam is usually added to the feed gas to evade falling in the explosion range and control the reaction temperature. However, the steam remains in the product gas and condenses in the absorption column, thereby lowering the concentration of product aqueous formaldehyde solution. The concentration of the aqueous formaldehyde solution thus obtained is about 45% by weight at highest.
On the other hand, the so called waste gas recycle process, in which a portion of the waste gas from the absorption column is added to the feed together with steam, has been proposed as an effective means for lowering the methanol content of the product aqueous formaldehyde solution and enhancing the concentration of formaldehyde. Even in that process, the practical formaldehyde concentration of the product aqueous formaldehyde solution is about 55% by weight at highest.
When a molar ratio of air to methanol in the feed gas is increased, combustion reaction of methanol, and other side reactions proceed, and the selectivity to formaldehyde is liable to be lowered. Thus, it is preferable to make the molar ratio of air to methanol as low as possible.